Likewise I use OS 10.3.x & Pro but don't have this issue. I have noticed the player is sometimes a little slow or stubborn to start. I just tried with 2 I'm presently dwnlding, both of which are over 100 MB in size but about 100 MB dwnlded already. It took the 1st one 20 secs to start, the 2nd 14 secs. Another which is 6% & a small file & not presently dwnlding started immediately. From my LW Library, I launched one & it started immediately. I think it can depend on how busy LW is at the time. I know on another day I tried to preview a file & it didn't appear to start. So I went to the library & launched one & it was fine, went back to the dwnld window & 2nd time that file worked ok.

Make sure your LW media player is still enabled. LW's Prefs>Player & tick option to enable player & press Apply.

I should also mention not to try to use the player until LW has settled down. If you have many incomplete & shared files, LW can take a considerable time to load & do its initial finding of sources by itself.

Oh & if you're using the Jum build, the player within that version has been disabled. But if you're using the regular Pro version you should be fine.